QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Full Text (PDF) All Versions of this Article: jpet.105.097006v1 317/2/910    most recent Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Vanover, K. E. Articles by Davis, R. E. Search for Related Content PubMed PubMed Citation Articles by Vanover, K. E. Articles by Davis, R. E.

NEUROPHARMACOLOGY

Pharmacological and Behavioral Profile of N-(4-Fluorophenylmethyl)-N-(1-methylpiperidin-4-yl)-N'-(4-(2-methylpropyloxy)phenylmethyl) Carbamide (2R,3R)-Dihydroxybutanedioate (2:1) (ACP-103), a Novel 5-Hydroxytryptamine_2A Receptor Inverse Agonist

ACADIA Pharmaceuticals Inc., San Diego, California (K.E.V., D.M.W., M.M., I.V., L.R.G., J.L., A.L.D.T., F.P., H.H.S., T.R.O., E.S.B., A.K.U., M.B.T., N.S., C.M.A., T.Y.S., S.C.H., B.-R.T., M.R.B., R.E.D.); and Department of Psychiatry, University of California, San Diego, La Jolla, California (S.B.P., M.A.G.)

Received October 14, 2005; accepted February 7, 2006.

	Abstract

Top Abstract Materials and Methods Results Discussion References

 
The in vitro and in vivo pharmacological properties of N-(4-fluorophenylmethyl)-N-(1-methylpiperidin-4-yl)-N'-(4-(2-methylpropyloxy)phenylmethyl)carbamide (2R,3R)-dihydroxybutanedioate (2:1) (ACP-103) are presented. A potent 5-hydroxytryptamine (5-HT)_2A receptor inverse agonist ACP-103 competitively antagonized the binding of [³H]ketanserin to heterologously expressed human 5-HT_2A receptors with a mean pK_i of 9.3 in membranes and 9.70 in whole cells. ACP-103 displayed potent inverse agonist activity in the cell-based functional assay receptor selection and amplification technology (R-SAT), with a mean pIC₅₀ of 8.7. ACP-103 demonstrated lesser affinity (mean pK_i of 8.80 in membranes and 8.00 in whole cells, as determined by radioligand binding) and potency as an inverse agonist (mean pIC₅₀ 7.1 in R-SAT) at human 5-HT_2C receptors, and lacked affinity and functional activity at 5-HT_2B receptors, dopamine D₂ receptors, and other human monoaminergic receptors. Behaviorally, ACP-103 attenuated head-twitch behavior (3 mg/kg p.o.), and prepulse inhibition deficits (1-10 mg/kg s.c.) induced by the 5-HT_2A receptor agonist (±)-2,5-dimethoxy-4-iodoamphetamine hydrochloride in rats and reduced the hyperactivity induced in mice by the N-methyl-D-aspartate receptor noncompetitive antagonist 5H-dibenzo[a,d]cyclohepten-5,10-imine (dizocilpine maleate; MK-801) (0.1 and 0.3 mg/kg s.c.; 3 mg/kg p.o.), consistent with a 5-HT_2A receptor mechanism of action in vivo and antipsychotic-like efficacy. ACP-103 demonstrated >42.6% oral bioavailability in rats. Thus, ACP-103 is a potent, efficacious, orally active 5-HT_2A receptor inverse agonist with a behavioral pharmacological profile consistent with utility as an antipsychotic agent.

Recently, it was reported that nearly all antipsychotics are 5-HT_2A receptor inverse agonists, in that they can attenuate the basal constitutive signaling activity of this receptor, in contrast to neutral antagonists that can only block agonist-induced responses and lack negative intrinsic efficacy (Weiner et al., 2001). Accordingly, a high-throughput 5-HT_2A receptor inverse agonist screen, followed by an effort in medicinal chemistry, was used to identify potent, selective, and highly efficacious 5-HT_2A receptor inverse agonists (Weiner et al., 2001). A prototype 5-HT_2A receptor inverse agonist, AC-90179, was shown to have efficacy in animal models for antipsychotic-like activity and lacked the side effects associated with other antipsychotic drugs, but it demonstrated poor oral bioavailability (Weiner et al., 2001; Vanover et al., 2004). Additional synthetic efforts designed to improve metabolic stability resulted in the discovery of ACP-103 (Fig. 1).

View larger version (7K): [in this window] [in a new window]   Fig. 1. Molecular structure of ACP-103.

Behaviorally, ACP-103 was tested for antipsychotic-like efficacy by measuring the ability of ACP-103 to attenuate (±)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI)-induced head twitches in rats. The induction of head twitches by the 5-HT₂ agonist DOI is thought to be mediated by brain 5-HT_2A receptors, and attenuation of DOI-induced head twitches is an activity shared by many atypical antipsychotic drugs (Wettstein et al., 1999). ACP-103 also was studied for its ability to restore a DOI-induced disruption of prepulse inhibition (PPI) of the acoustic startle response, an experimental paradigm that measures sensorimotor gating processes in rats. This test is based on the observation that a high-intensity stimulus (e.g., an abrupt loud noise) causes a startle response and that when a stimulus of lesser intensity (e.g., a quieter noise) occurs shortly before the high-intensity stimulus, the startle response is diminished. This sensorimotor-gating process is disrupted in schizophrenia patients (Braff et al., 1978). Atypical antipsychotic drugs that have high affinity for 5-HT_2A receptors have been shown to restore the DOI-induced PPI deficits in rats (Geyer et al., 2001). To extend the antipsychotic-like behavioral profile beyond direct 5-HT_2A receptor interactions, ACP-103 was tested for its ability to attenuate hyperactivity induced by the noncompetitive N-methyl-D-aspartate (NMDA) antagonist MK-801 (dizocilpine maleate). Blockade of NMDA antagonist-induced hyperactivity is predictive of antipsychotic-like efficacy (Freed et al., 1984). It was hypothesized that ACP-103 would attenuate DOI-induced head twitches and PPI deficits as well as MK-801-induced hyperactivity. Lastly, the pharmacokinetic profile of ACP-103 was evaluated in rats.

	Materials and Methods

Top Abstract Materials and Methods Results Discussion References

 
In Vitro Pharmacological Experiments
Receptor Cloning. Full-length sequence-verified clones of each of the three human 5-HT₂ receptor subtypes were isolated and subcloned into a mammalian expression vector as described previously (Weiner et al., 2001). The 5-HT_2C receptor is naturally subject to RNA editing, which alters the protein sequence of the receptor, generating a multitude of functionally distinct isoforms (Burns et al., 1997). Site-directed mutagenesis was used to create the various 5-HT_2C receptor isoforms as described previously (Marion et al., 2004). All 5-HT_2C receptor selection and amplification technology (R-SAT) inverse agonist experiments were performed with the constitutively active INI isoform, whereas radioligand-binding experiments utilized the VGV isoform (membranes) or the INI isoform (whole cells). The expression levels for the cells were as follows: 530 ± 264 fmol/mg protein (n = 18) for 5-HT_2A, 3251 ± 1457 fmol/mg protein (n = 20) for 5-HT_2C, and 277 ± 139 fmol/mg protein (n = 4) for 5-HT_2B.

Receptor Selection and Amplification Technology. R-SAT assays were performed as described previously (Weiner et al., 2001), with the following modifications. In brief, NIH-3T3 cells were grown to 80% confluence in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% bovine calf serum (Hyclone Laboratories, Logan, UT) and 1% penicillin/streptomycin/glutamine (Invitrogen, Carlsbad, CA). Cells were transfected in roller bottles for 18 h with the relevant G protein-coupled receptor gene and the gene for -galactosidase. After transfection, cells were trypsinized, harvested, and frozen. Aliquots of frozen cell batches were thawed and tested for response to reference agonists and inverse agonists ensuring pharmacologically appropriate responses. To initiate an assay, cells were thawed rapidly and prepared in DMEM contained 0.4% calf serum (Hyclone Laboratories), 30% UltraCulture (BioWhittaker, Rockland, ME), and 1% penicillin/streptomycin/glutamine, and then they were added to half-area 96-well microtiter plates containing either test compounds or reference ligands. After 5 days in culture, media were removed from the wells, and the cells were incubated at room temperature in 200 µl of phosphate-buffered saline, pH 7.4, with 3.5 mM o-nitrophenyl--D-galactopyranoside (Sigma-Aldrich, St. Louis, MO) and 0.5% Nonidet P-40 (Sigma-Aldrich). After 2 to 4 h, the plates were read at 420 nm on a plate-reader (Bio-Tek Instruments, Winooski, VT).

Radioligand Binding. For the membrane binding, NIH-3T3 cells were grown to 70% confluence in 15-cm² dishes and transfected with 10 µg of receptor plasmid DNA using Polyfect transfection reagent (QIAGEN, Valencia, CA) according to manufacturer's protocols. Two days after transfection, cells expressing the desired serotonin receptor were homogenized in 20 mM HEPES/10 mM EDTA and spun down at 11,000g at 4°C for 30 min. The supernatant was discarded, and the pellet was resuspended in 20 mM HEPES/1 mM EDTA and spun down at the same setting. The pellet was then resuspended in 20 mM HEPES/0.5 mM EDTA, and membranes were used for binding assays. Bradford analysis was used to determine total membrane protein. K_d and B_max values were derived from 12-point concentration experiments using 1 nM [³H]ketanserin (PerkinElmer Life and Analytical Sciences, Boston, MA) for the 5-HT_2A receptor and 3 nM [³H]mesulergine (Amersham Biosciences, Piscataway, NJ) for the 5-HT_2B and 5-HT_2C receptors. Membranes were incubated at room temperature for 3 h with various concentrations of test ligand in the presence of a fixed concentration of radioligand. The suspension was filtered as explained below for whole-cell binding, washed with ice-cold buffer, and dried, and radioactivity was determined using TopCount (Packard Bioscience, Shelton, CT).

For the whole-cell binding, 6 million human embryonic kidney 293T cells were plated in 10-cm dishes and transfected with 5 µg of plasmid DNA using Polyfect according to manufacturer's instructions. Two days after transfection, cells were harvested with 10 mM EDTA, washed, and resuspended in binding buffer (1x DMEM with 0.1% bovine serum albumin). Then, 60,000 cells transfected with the 5-HT_2A receptor or 20,000 cells transfected with the 5-HT_2C-INI receptor were incubated at 37°C for 3 h in the presence of 5 nM radioligand ([³H]ketanserin for 5-HT_2A receptors and [³H]mesulergine for 5-HT_2C-INI receptors) and varying concentrations of ligands (total volume 100 µl in a 96-well plate). Cells were filtered onto a 96-well GF/B filter plate (Packard Bioscience) and washed with 300 ml of wash buffer (25 mM HEPES, 1 mM CaCl₂, 5 mM MgCl₂, and 0.25 M NaCl) using a Filtermate 196 harvester (Packard Instruments, Downers Grove, IL). The filter plates were dried under a heat lamp before addition of 50 µl of scintillation fluid to each well (Microscint 20; Packard Bioscience). Plates were counted on a TopCount (Packard Bioscience). Separately, the hydrochloride salt form of ACP-103 (10 µM) was evaluated at MDS Pharma Services (Taipei, Taiwan) for activity in a broad screen of radioligand binding assays at 65 different receptors.

Behavioral Experiments
Animals and Apparatus. Male Sprague-Dawley rats, male non-Swiss albino mice (Harlan, San Diego, CA) were used as subjects. Rats weighed 200 to 300 g, and mice weighed 20 to 30 g. Animals were housed two per cage (rats) or eight per cage (mice) in a room with controlled temperature and a 12-h light/dark cycle. Water and standard rodent chow (Harlan Teklad, Madison, WI) were continuously available in the home cage.

The startle testing was performed as reported previously (Mansbach et al., 1988) in commercially available startle chambers (San Diego Instruments, San Diego, CA). Plastic locomotor activity cages (20 x 20 x 30 cm; AccuScan Instruments, Columbus, OH) were equipped with photocell beams for monitoring horizontal activity. Data were collected using Versamax computer software (AccuScan Instruments). Immediately before locomotor testing, mice were evaluated for myorelaxation/ataxia using a custom-built apparatus consisting of a metal wire (2 mm in diameter) suspended 25 cm above the benchtop.

Procedure. For DOI head-twitch experiments in rats, vehicle or a dose of ACP-103 was administered orally 120 min before DOI administration. DOI HCl (2.5 mg/kg i.p.) was administered immediately before observations. After injection of DOI, each rat was placed into an empty cage and observed. Latency to the first head twitch and the number of head twitches occurring over 5 min were recorded. Each rat was used only once with eight to 16 rats per dose group.

The PPI of acoustic startle response experiments were conducted in rats at the University of California San Diego. Rats first were matched for levels of acoustic startle magnitude and PPI and assigned to the drug treatment groups. Two days after the baseline session, rats were tested again in the startle chambers to assess the effects of ACP-103 and the 5-HT_2A receptor antagonist MDL-100,151 (racemic M100907, as a positive control) on DOI-induced disruptions of PPI. The session used to assess drug effects consisted of a 5-min acclimation period in the chamber with a constant background noise (65 dB) followed by 62 presentations of acoustic stimuli and 100 subsequent presentations of air puffs to measure acoustic and tactile startle responses, respectively. The 62 acoustic trials consisted of 24 40-ms presentations of a 120-dB broadband pulse, 10 20-ms presentations of each prepulse intensity (68, 71, and 77 dB) 100 ms before a 40-ms presentation of a 120-dB broadband pulse, and eight no-stimulus trials in which no acoustic pulse was delivered to assess general motor activation in the rats. Five 120-dB trials, which were not included in the calculation of PPI values, were presented at the beginning of the test session to achieve a relatively stable level of startle reactivity for the reminder of the session, based on the observation that the most rapid habituation of the startle reflex occurs within the first few presentations of the startling stimulus (Geyer et al., 1990). Another five 120-dB trials, also not included in the calculation of PPI values, were presented at the end of the test session to assess startle habituation. At the end of the 62 acoustic trials, there was a 1-min exposure to a constant 65-dB background noise followed by 100 30-ms presentations of 40-psi air puffs. Acoustic trials were presented in a pseudorandom order with an average intertrial interval of 15 s (range 7-23 s), and air puffs were presented every 10 s.

Thirty minutes before being placed in the startle apparatus, rats were treated with saline (s.c.), MDL-100,151 (1.0 mg/kg s.c.), or one of three doses of ACP-103 (1.0, 3.0, or 10.0 mg/kg s.c.). Five minutes after the pretreatment, rats were administered either DOI HCl (0.5 mg/kg s.c.) or 0.9% saline (s.c.). The acoustic startle session lasted approximately 37 min. After 1 week, rats were tested again in the same acoustic/tactile startle session in the exact order and at the same time as the previous week. The same pretreatment drug or vehicle was administered, and rats were crossed over to receive the treatment opposite to that they received the previous week (e.g., DOI HCl for week 1, 0.9% saline for week 2).

Non-Swiss albino mice were used for locomotor activity experiments. For determination of spontaneous activity, ACP-103 was administered alone (s.c. 60 min before session start or p.o. 60 min before session start). For hyperactivity experiments, mice were treated with 0.3 mg/kg MK-801 (i.p.) 15 min presession (the peak dose for producing hyperactivity in an inverted-U dose-effect curve as determined in pilot experiments) in combination with vehicle or ACP-103. Motor activity data were collected during a 15-min session in a lit room. Mice had no prior exposure to the motor cages. Immediately before placing the mice in the locomotor chambers, effects on myorelaxation/ataxia were determined by placing each of the mouse's forepaws in contact with a horizontal wire while holding the mouse by the base of the tail. Mice were required to bring at least one hindpaw in contact with the wire within 10 s to be scored as a "pass" and failure to do so was considered ataxic. Each dose or dose combination was tested in a separate group of mice (n = 8).

Data Analysis. All pharmacological data were analyzed using Prism 4 for Windows (GraphPad Software Inc., San Diego, CA) and are reported as averaged values derived from multiple experimental replicates (n) ± S.D. Inverse agonist potencies are reported as pIC₅₀ values, whereas affinity measurements derived from radioligand binding experiments are reported as pK_i values that were corrected according to the formula K_i = (IC₅₀)/(1 + [L]/K_d), in which [L] is the concentration of the test compound, and K_d is the dissociation constant of the radioligand.

For DOI head-twitch experiments, latency (seconds) to the first head twitch and the number of head twitches were recorded and averaged across animals in a group. For locomotor experiments, distance traveled (centimeters) was calculated and averaged across animals in a group. Means ± S.E.M. were calculated. An analysis of variance and post hoc Dunnett's t test comparisons to vehicle control were conducted for each dose-response function.

For PPI experiments, startle magnitude was quantified from the PULSE-ALONE (120-dB) trials and the PREPULSE + PULSE trials. Percentage of prepulse inhibition (%PPI) for the three prepulse intensities was calculated according to the following formula: %PPI = 100 - {[(startle response for PREPULSE + PULSE trial)/(startle response for PULSE-ALONE trial)] x 100}. A three-way repeated measures analysis of variance with pretreatment as a between-subject factor and treatment and prepulse intensity as within-subject factors was performed on the %PPI data. A separate three-way repeated measures analysis of variance was performed on the %PPI data from the MDL-100,151 comparison. Because there was pretreatment by treatment interactions in both analyses of variance, subsequent two-way repeated measures analyses of variance were performed on the DOI and vehicle-treated groups. The level was adjusted to 0.025 for the two-way pairwise repeated measures analyses of variance, since one within-subject factor was dropped from the analysis. The same analyses were performed on the acoustic startle data (startle to the PULSE alone trials) with pretreatment as a between-subject factor and treatment and time as within-subject factors.

Pharmacokinetics
The oral bioavailability of a hydrochloride salt form of ACP-103 was determined at Absorption Systems (Exton, PA) in male Sprague-Dawley rats (Hilltop Lab Animals, Scottdale, PA) weighing between 280 and 310 g. Samples were withdrawn at the following time points: for intravenous administration, 0 (predose), 2, 5, 30, 60, 120, 240, 360, and 480 min; and for oral administration, 0 (predose), 5, 30, 60, 120, 240, 360, and 480 min. Approximately 0.50 to 0.75 ml of whole blood was collected by venipuncture from the jugular vein. The blood was transferred to heparinized tubes and placed on ice until centrifuged. After centrifugation, the plasma was placed on ice until frozen at -70°C before shipment to the analytical laboratory.

To determine the concentration of ACP-103 (hydrochloride form) in rat plasma samples, standards were prepared with rat plasma in sodium citrate obtained from Lampire Biological Laboratories (Pipersville, PA) (lot number 062631954) to contain 1000, 300, 100, 30, 10, 3, 1, and 0.3 ng/ml ACP-103 (hydrochloride). Plasma standards were treated identically to the plasma samples. Plasma samples were prepared by protein precipitation. A 50-µl aliquot of plasma was combined with 150 µl of acetonitrile. The mixture was then spiked with 10 µl of internal standard, vortexed, and centrifuged for 10 min at 10,000 rpm. The supernatant (180 µl) was then evaporated to dryness and reconstituted with 100 µl of 66% acetonitrile and 44% deionized water. After vortexing and a second centrifugation at 10,000 rpm for 10 min, the sample was injected into the liquid chromatograph/tandem mass spectrometer. The concentration of the internal standard solution was 2 µg/ml AC-90179. Pharmacokinetic analysis was performed on the average plasma concentration for each time point. The data were subjected to noncompartmental analysis using the pharmacokinetic program WinNonlin version 3.1 (Pharsight, Mountain View, CA).

Drugs. ACP-103, eplivanserin, and M100907 and its racemate MDL-100,151 were synthesized by ACADIA Pharmaceuticals Inc. Ritanserin, haloperidol, MK-801, and DOI were obtained from Sigma-Aldrich, and clozapine was obtained from Tocris Cookson Inc. (Ellisville, MO). For the behavioral experiments, ACP-103, MDL-100,151, DOI, and MK-801 were dissolved in 0.9% saline, and haloperidol was dissolved in 10% Tween 80 (90% water). All compounds were administered in a volume of 0.1 ml/10 g body weight, and doses were calculated based on the weight of the salt. For the contracted radioligand binding experiments at MDS Pharma Services, a hydrochloride salt form of ACP-103 was used. For the pharmacokinetic experiment, a solution of the hydrochloride salt was prepared using 5% dextrose (w/v) in aqueous polyethylene glycol-400 [4:1 (v/v)].

	Results

Top Abstract Materials and Methods Results Discussion References

 
Pharmacology Experiments
Functional Antagonism and Inverse Agonism. Transient expression of the three human 5-HT₂ receptors in NIH-3T3 cells as part of the R-SAT assay generates robust and pharmacologically appropriate receptor-mediated responses (Fig. 2; Table 1). Each of these receptors displays detectable basal levels of constitutive activity that allows for the characterization of the potency and efficacy of 5-HT₂ receptor inverse agonists. Figure 2 displays representative concentration-response curves for the nonselective 5-HT₂ receptor inverse agonist ritanserin, as well as those observed for ACP-103 (Fig. 1), the 5-HT_2A receptor-selective compound M100907 (Kehne et al., 1996), eplivanserin (Rinaldi-Carmona et al., 1992), and the atypical antipsychotic clozapine, at 5-HT_2A (Fig. 2A) and 5-HT_2C (Fig. 2B) receptors. Each of these compounds was found to be potent (pIC₅₀ values greater than 8.0) inverse agonists at the 5-HT_2A receptor (Fig. 2; Table 1). The rank order of potency of these compounds at the 5-HT_2A receptor was ritanserin > M100907 = eplivanserin = ACP-103 > clozapine. As expected, all compounds were found to competitively antagonize serotonin-induced 5-HT_2A receptor agonist responses in R-SAT (data not shown). Regarding efficacy, all compounds displayed full inverse agonist efficacy (75% or greater) relative to ritanserin (Table 1). In contrast, only ritanserin and clozapine displayed potent inverse agonist activity at the human 5-HT_2B receptor (Table 1). The inverse agonist potencies of these compounds at the 5-HT_2C receptor were significantly less than those observed at 5-HT_2A receptors. As depicted in Fig. 2B, ritanserin, ACP-103, M100907, and clozapine displayed inverse agonist activity; however, eplivanserin was found to lack negative intrinsic efficacy. The rank order of potency at the 5-HT_2C receptor was ritanserin > ACP-103 > clozapine > M100907 (Table 1). Each of these compounds displayed full inverse agonist efficacy (Table 1). All five compounds were found to competitively antagonize serotonin-induced 5-HT_2C receptor agonist responses in R-SAT (data not shown). Finally, ACP-103 was tested against 31 of the total 36 human monoaminergic receptors, including 5-HT_1A, 5-HT_1B, 5-HT_1D, 5-HT_1E, 5-HT_1F, 5-HT₃, 5-HT_6A, and 5-HT_7A, and was found to lack functional activity (pEC₅₀ or pK_i values less than 6.0) at the 31 monoaminergic receptors and additional five nonmonoaminergic (somatostatin SST₁, SST₂, SST₃, SST₄, and SST₅) receptor subtypes when tested as an agonist, inverse agonist, or a competitive antagonist in R-SAT or other functional assays (data not shown).

View larger version (17K): [in this window] [in a new window]   Fig. 2. Inverse agonism of ACP-103 at human 5-HT2A and 5-HT2C receptors. Representative concentration-response curves for ACP-103 and various 5-HT2 compounds as inverse agonists at the human 5-HT2A (A) and 5-HT2C (B) receptors, as determined by R-SAT assays, are reported. Compounds were tested at eight different concentrations where each point was assayed in duplicate, with error bars depicting the mean ± S.E. Data are reported as percentage of inhibition of basal responses, where -100% represents the maximal response obtained for the reference inverse agonist ritanserin. Compounds tested include ritanserin (filled circles), ACP-103 (filled squares with dashed lines), M100907 (filled diamonds), eplivanserin (filled triangles), and clozapine (open circles). Observed 5-HT2A receptor inverse agonist potencies (pIC50 values) in the reported experiments (A) were ritanserin (10.1), ACP-103 (8.9), M100907 (9.2), eplivanserin (9.1), and clozapine (8.3). Observed 5-HT2c receptor inverse agonist potencies (pIC50 values) in the reported experiments (B) were ritanserin (8.4), ACP-103 (7.3), M100907 (6.3), eplivanserin (no activity), and clozapine (7.0).

Radioligand Binding. To further characterize the pharmacology of ACP-103 at human 5-HT₂ receptors, affinity determinations were made using radioligand binding techniques in membranes and whole cells (Table 1). All five compounds displayed high affinity for both receptors. The rank order of affinity for the 5-HT_2A receptor was M100907 = eplivanserin = ritanserin > ACP-103 = clozapine for membrane binding and M100907 = ACP 103 > eplivanserin > ritanserin > clozapine for whole-cell binding. The rank order of affinity at the 5-HT_2C receptor was ritanserin > clozapine = eplivanserin = ACP-103 > M100907 for membrane binding and ACP-103 > ritanserin > M100907 = clozapine = eplivanserin for whole-cell binding. The observed affinity values were similar to the observed functional potencies as inverse agonists. Eplivanserin was found to possess high affinity for the 5-HT_2C receptor (Table 1) to competitively antagonize serotonin-induced functional responses (data not shown) yet lack inverse agonist activity (Fig. 2B; Table 1), consistent with its designation as a competitive (neutral) antagonist at human 5-HT_2C receptors. ACP-103 was found to lack affinity (pK_i < 6.0) for the human dopamine D₂ receptor as determined by its inability to competitively displace spiperone or raclopride in radioligand binding assays (data not shown). Finally, a broad receptor binding profiling screen conducted at MDS Pharma Services that included a range of channels, enzymes, and receptors confirmed that, of 65 assays, the only high-affinity (pK_i > 7.0) interaction of ACP-103 was at 5-HT₂ receptors (Table 2).

View this table: [in this window] [in a new window]   TABLE 2 Broad screen receptor profiling for ACP-103 in radioligand binding If ACP-103 (10 µM) demonstrated greater than 50% inhibition in receptor binding, the percentage of inhibition is listed in the table. The IC50 and Ki values were only determined at those receptors for which 10 µM ACP-103 demonstrated 100% inhibition. ACP-103 (10 µM) demonstrated less than 50% inhibition in receptor binding assays at the following receptors: adenosine A1, adenosine A2A, adenosine A2B, adrenergic 1A, adrenergic 1B, adrenergic 1D, adrenergic 2A, adrenergic 2B, adrenergic 2, bradykinin B1, bradykinin B2, calcium channel type N, dopamine D1, dopamine D2L, dopamine D4,2, dopamine transporter, endothelin ETA, endothelin ETB, epidermal growth factor (EGF), estrogen ER, GABA transporter, GABAA agonist site, GABAA benzodiazepine central, GABAB, glucocorticoid, glutamate kainate, glutamate NMDA agonism, glutamate NMDA glycine, glutamate NMDA phencyclidine, histamine H1 central, histamine H2, histamine H3, imidazoline I2 central, interleukin IL-1, leukotriene B4, leukotriene D4, neuropeptide Y1, neuropeptide Y2, nicotinic acetylcholine central, opiate , opiate , opiate µ, phorbol ester, platelet-activating factor (PAF), potassium channel [KATP], purinergic P2X, purinergic P2Y, serotonin 5-HT1A, serotonin 5-HT3, serotonin 5-HT4, serotonin transporter, tachykinin NK1, and testosterone.

Behavioral Experiments
DOI (2.5 mg/kg i.p.) with a vehicle pretreatment induced an average of 5.2 ± 0.9 (S.E.M.) head twitches with the latency to head-twitch onset of 183 ± 25.3 s. ACP-103 (n = 8-16/group) attenuated DOI-induced head twitches at a dose of 3 mg/kg p.o. after a 2-h pretreatment (Fig. 3). The analysis of variance for number of head twitches [F(5,71) = 2.795; p = 0.02] was statistically significant, and for latency [F(5,71) = 1.970; p = 0.09], it was a statistical trend. The number of head twitches after DOI plus 3 mg/kg ACP-103 was 0.75 ± 0.27. The latency to first head twitch after DOI plus 3 mg/kg ACP-103 was 264.9 ± 11.4 s. No effect on head-twitch behavior was seen after treatment with ACP-103 over the same dose range alone (data not shown).

View larger version (19K): [in this window] [in a new window]   Fig. 3. Attenuation of DOI-induced head twitches by ACP-103. DOI (2.5 mg/kg i.p.) was administered in combination with vehicle (veh) or various doses of ACP-103. A, number of head twitches produced by DOI as a function of ACP-103 dose. B, latency in seconds to the first head twitch as a function of ACP-103 dose. Each bar represents the mean + S.E.M. from a separate group of animals (n = 6/dose). Asterisks indicate statistical significance (p < 0.05) compared with DOI + vehicle.

View larger version (21K): [in this window] [in a new window]   Fig. 4. Attenuation of DOI-induced PPI by ACP-103. DOI (0.5 mg/kg s.c.) was administered in combination with vehicle (veh), various doses of ACP-103, or 1.0 mg/kg MDL-100,151 (racemic M100907) (s.c.). %PPI is shown as a function of ACP-103 dose + S.E.M. The insert shows that ACP-103 had no effect on basal startle response. Each bar represents the mean + S.E.M. Asterisks indicate statistical significance (p < 0.05) compared with DOI + vehicle.

ACP-103 significantly attenuated MK-801-induced hyperactivity in mice at doses of 0.1 and 0.3 mg/kg s.c. [F(7,63) = 6.010; p < 0.0001; Fig. 5A], consistent with an antipsychotic-like effect. ACP-103 also reduced spontaneous activity at similar doses when administered s.c. [F(7,63) = 2.741; p = 0.0161]. All mice were able to pull their hindpaws up to the horizontal wire after administration of ACP-103, which was tested just before placing the mice in the locomotor chamber. In a separate experiment, seven of eight mice were able to pull their hindpaws up to the horizontal wire after administration of 100 mg/kg s.c. ACP-103 alone, indicating a lack of ataxia even at a very high dose.

View larger version (18K): [in this window] [in a new window]   Fig. 5. Effects of ACP-103 on spontaneous locomotion and MK-801-induced hyperactivity in mice. Distance traveled is shown as a function of ACP-103 dose. A, effects of ACP-103 after s.c. administration. B, effects of ACP-103 after p.o. administration. Each point represents the mean and S.E. of a separate group of mice (n = 8/group). Open squares represent spontaneous locomotion, whereas filled squares represent MK-801 hyperactivity. Asterisks indicate statistical significance (p < 0.05) compared with respective vehicle control.

Pharmacokinetics
ACP-103 exhibits a volume of distribution much larger than the total body water of the rat, indicating extensive partitioning into fatty tissues (Table 3). The high systemic clearance was consistent with the 59-min half-life observed. Considering that the area under the curve after 10 mg/kg is truncated, the oral bioavailability at this dose is higher than 42.6%. After oral administration, ACP-103 showed the elimination phase of the compound with maximal plasma concentrations occurring 60 to 120 min postdosing.

	Discussion

Top Abstract Materials and Methods Results Discussion References

 
A variety of compounds were characterized based on their 5-HT₂ receptor affinity, as determined by competitive radioligand binding assays, and their potency and efficacy as 5-HT₂ receptor inverse agonists using the cell-based functional assay R-SAT. Not surprisingly, all five compounds potently displaced the binding of [³H]ketanserin to 5-HT_2A receptors in both membranes and whole cells, where the absolute and rank order of affinities were similar to those reported previously by our group and others (Leysen et al., 1985; Rinaldi-Carmona et al., 1992; Kehne et al., 1996; Bonhaus et al., 1999; Weiner et al., 2001). Interestingly, the absolute affinities for most compounds were higher in the membrane preparation than in whole cells. Only ritanserin and clozapine had appreciable affinity for the 5-HT_2B receptor, consistent with prior reports (Bonhaus et al., 1999; Weiner et al., 2001). In contrast, all compounds competitively antagonized the binding of [³H]mesulergine to the VGV isoform of the human 5-HT_2C receptor in membranes and the INI isoform in whole cells. The affinities observed at the 5-HT_2C receptor for ritanserin were similar to those observed by Bonhaus et al. (1999). The whole-cell binding affinity for clozapine at the 5-HT_2C receptor was similar to that reported previously (Canton et al., 1990; Roth et al., 1992; Herrick-Davis et al., 2000), whereas our observed 5-HT_2C receptor affinity for clozapine in membranes was approximately 100-fold lower. The affinity observed for M100907 was slightly higher than described previously (Kehne et al., 1996). Receptor selectivity for 5-HT_2A receptors over 5-HT_2C receptors depends on the assay, but rank orders for selectivity of 5-HT_2A receptors were generally in agreement across the assays. For all compounds, the physiologically relevant assays (the R-SAT functional assay and the whole-cell binding assay) showed greater absolute selectivity than the membrane binding assay. Overall, M100907 displayed the highest 5-HT_2A/5-HT_2C receptor selectivity among this group of compounds, where ACP-103 displayed a 5-HT_2A/5-HT_2C receptor affinity ratio similar to clozapine. The 5-HT_2A/5-HT_2C receptor selectivity profiles of ACP-103 and clozapine were similar to that of olanzapine published previously, but in contrast to ACP-103, olanzapine showed similar activity at 5-HT_2B receptors as clozapine (Weiner et al., 2001). Risperidone was more potent at 5-HT_2A receptors than at 5-HT_2B receptors and showed no activity at 5-HT_2C receptors (Weiner et al., 2001).

Interestingly, all of the tested compounds demonstrated inverse agonism or negative intrinsic activity at 5-HT_2A receptors. Without potent neutral antagonists to test for comparison, it is unclear whether functional differences between inverse agonism and neutral antagonism can be demonstrated in vivo. Furthermore, all antipsychotic drugs with 5-HT_2A receptor affinity are inverse agonists (Weiner et al., 2001). Whether inverse agonism is necessary for antipsychotic efficacy or not remains uncertain. Given that ACP-103 shares the 5-HT2A receptor inverse agonism with effective antipsychotic drugs, ACP-103 is likely to share any efficacy that is imparted by such a mechanism.

Of the compounds tested, only eplivanserin lacked negative intrinsic activity at 5-HT_2C receptors, a property shared by a small number of known antipsychotics (Weiner et al., 2001). The clinical significance of 5-HT_2C receptor inverse agonism is currently unknown. The expression of 5-HT_2C receptors in dopaminergic neurons in the central nervous system (Eberle-Wang et al., 1997) and the neurochemical effects of 5-HT_2C receptor blockade (Di Matteo et al., 2001) suggest a utility in modifying dopaminergic function, a property that may be advantageous in the treatment of psychosis (Herrick-Davis et al., 2000), Parkinson's disease (Eberle-Wang et al., 1996), affective disorders (Cremers et al., 2004), and sleep (Frank et al., 2002). Given the relative selectivity for ACP-103 at 5-HT_2A receptors over 5-HT_2C receptors, it is reasonable to suggest that ACP-103 might show selective blockade for 5-HT_2A receptors at low doses, whereas blockade of both receptor subtypes may be possible at higher doses. If the drug has a high degree of safety, the potential therapeutic benefits of 5-HT_2C receptor inverse agonism, in addition to 5-HT_2A receptor inverse agonism, may be obtained by treatment with ACP-103. The present experiments do not address the issue of receptor selectivity in vivo. The challenges of doing so are emphasized by the lack of availability of selective 5-HT_2A or 5-HT_2C receptor agonists that can be used as tools in conjunction with ACP-103 to understand the in vivo selectivity of ACP-103. Furthermore, the lack of commercially available selective radioligands for 5-HT_2A or 5-HT_2C receptors does not allow for the demonstration of selectivity of this compound ex vivo. Thus, additional nonclinical pharmacology experiments as well as the clinical test of the hypotheses are warranted, but not straightforward due to the lack of appropriate tools, and they are outside of the scope of the present study.

To determine the potential in vivo antipsychotic-like activity, ACP-103 was evaluated in several animal models that may predict antipsychotic drug efficacy in humans. ACP-103 potently and robustly attenuated DOI-induced head twitches in rats, with a minimal effective oral dose of 3.0 mg/kg. Interestingly, this effect was not seen at a higher dose. Whether this was due to other activity, perhaps 5-HT_2C, or variability at the higher dose is unclear. ACP-103 also reversed DOI-induced PPI deficits in rats at doses between 1.0 and 10.0 mg/kg. These doses of ACP-103 did not alter the basal startle response. The reversals of the DOI-induced PPI deficits and head twitches demonstrate that ACP-103 can inhibit the actions of a direct 5-HT_2A receptor agonist, consistent with a 5-HT_2A mechanism of action in vivo. To extend the antipsychotic-like behavioral profile beyond direct 5-HT_2A receptor interactions, ACP-103 attenuated hyperactivity induced by the noncompetitive NMDA antagonist MK-801, with a minimal effective dose of 0.1 mg/kg s.c. or 3 mg/kg p.o. These effects are consistent with previous reports that other 5-HT_2A receptor antagonists and inverse agonists, as well as atypical antipsychotics, have been shown to reduce MK-801- and phencyclidine-induced behavioral effects (Freed et al., 1984; Carlsson et al., 1999b; Weiner et al., 2001; Vanover et al., 2004).

Because of the selectivity at 5-HT_2A receptors, ACP-103 was predicted to have an improved side effect profile relative to other antipsychotic drugs. Although ACP-103 decreased spontaneous locomotor behavior after s.c. administration, ACP-103 did not exhibit any myorelaxant or ataxic effects in those same mice as measured by the horizontal wire test, and ACP-103 did not reduce spontaneous locomotion at efficacious doses after oral administration. These results would predict less sedation by ACP-103 relative to other antipsychotic drugs in humans.

Clinical data from investigations of 5-HT_2A receptor-selective compounds, including M100907 (Potkin et al., 2001) and eplivanserin (Meltzer et al., 2004), suggest that they are clinically efficacious compared with placebo in the treatment of acute exacerbation of schizophrenia. In addition, 5-HT_2A receptor inverse agonists may find clinical utility as combination therapy with existing antipsychotic drugs. Given that dopamine D₂ receptor antagonism causes extrapyramidal motor side effects and tardive dyskinesia, many antipsychotic drugs are contraindicated for use in sensitive populations such as Parkinson's disease patients. Indeed, the low doses of clozapine that have been shown to be effective against treatment-induced psychosis and tolerated motorically in Parkinson's disease (The French Clozapine Study Group, 1999; The Parkinson Study Group, 1999) are thought to produce high levels of 5-HT_2A receptor occupancy and little D₂ receptor occupancy (Nordstrom et al., 1993; Meltzer et al., 1995). These observations suggest that selective 5-HT_2A receptor inverse agonists may be therapeutically beneficial for treatment-induced psychosis in Parkinson's disease (Weiner et al., 2003).

ACP-103 was orally bioavailable in rats. The high oral bioavailability at 10 mg/kg p.o. seems to be consistent with the behavioral efficacy observed after oral administration of ACP-103 in the animal models. A high volume of distribution and high clearance for ACP-103 also were observed in the pharmacokinetic experiment.

ACP-103 shows antipsychotic-like efficacy in animal models consistent with 5-HT_2A receptor mediation. The behavioral effects of ACP-103 are similar to those observed with clozapine (Vanover et al., 2004). Because mice treated with ACP-103 were not sedated or ataxic, ACP-103 is unlikely to exacerbate the motor symptoms in schizophrenic or more susceptible populations such as Parkinson's disease patients. This profile differs from other antipsychotic drugs used to treat psychosis in Parkinson's disease, such as olanzapine, that exacerbate motor symptoms (Breier et al., 2002). Together, ACP-103 may address the need for an orally bioavailable and selective 5-HT_2A receptor inverse agonist for the treatment of psychosis in Parkinson's disease as well as in schizophrenia.

	Acknowledgements

 
We thank the following for excellent technical assistance and G. Anderson, K. Duncan, D. Hubbard, R. Johnson, K. Schurb, R. Zamora, and M. Buell. M.A.G. holds an equity interest in San Diego Instruments.

	Footnotes

 
Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.

doi:10.1124/jpet.105.097006.

ABBREVIATIONS: 5-HT, 5-hydroxytryptamine; AC-90179, 2-(4-methoxyphenyl)-N-(4-methylbenzyl)-N-(1-methylpiperidin-4-yl)acetamide, hydrochloride; ACP-103, N-(4-fluorophenylmethyl)-N-(1-methylpiperidin-4-yl)-N'-(4-(2-methylpropyloxy)phenylmethyl) carbamide (2R,3R)-dihydroxybutanedioate (2:1); DOI, (±)-2,5-dimethoxy-4-iodoamphetamine hydrochloride; PPI, prepulse inhibition; NMDA, N-methyl-D-aspartate; MK-801, 5H-dibenzo[a,d]cyclohepten-5,10-imine (dizocilpine maleate); R-SAT, receptor selection and amplification technology; DMEM, Dulbecco's modified Eagle's medium; M100907, R-(+)--(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol; SST, somatostatin; MDL-100,151, (±)--(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol.

¹ Current affiliation: TorreyPines Therapeutics, La Jolla, California.

² Current affiliation: Amgen, Cambridge, Massachusetts.

³ Current affiliation: Haldor Topsoe, Vaerloese, Denmark.

⁴ Current affiliation: Department of Chemistry, KaroBio AB, NOVUM, Huddinge, Sweden.

⁵ Current affiliation: Sharp Memorial Hospital, San Diego, California.

Address correspondence to: Dr. Kimberly E. Vanover, ACADIA Pharmaceuticals Inc., 3911 Sorrento Valley Blvd., San Diego, CA 92121. E-mail: kvanover{at}acadia-pharm.com

	References

Top Abstract Materials and Methods Results Discussion References

 

Breier A, Sutton VK, Feldman PD, Kadam DL, Ferchland I, Wright P, and Friedman JH (2002) Olanzapine in the treatment of dopamimetic-induced psychosis in patients with Parkinson's disease. Biol Psychiatry 52: 438-445.[CrossRef][Medline]

Bonhaus DW, Bach C, DeSouza A, Salazar R, Matsuoka BD, Zuppan P, Chan HW, and Eglen RM (1999) The pharmacology and distribution of human 5-hydroxytryptamine_2B (5-HT_2B) receptor gene products: comparison with 5-HT_2A and 5-HT_2C receptors. Br J Pharmacol 115: 622-628.

Burns CM, Chu H, Rueter SM, Hutchinson LK, Canton H, Sanders-Bush E, and Emerson RB (1997) Regulation of serotonin-2C receptor G-protein coupling by RNA editing. Nature (Lond) 387: 303-308.[CrossRef][Medline]

Braff D, Stone C, Callaway E, Geyer M, Glick I, and Bali L (1978) Prestimulus effects on human startle reflex in normals and schizophrenics. Psychophysiology 15: 339-343.[Medline]

Canton H, Verriele L, and Colpaert FC (1990) Binding of typical and atypical antipsychotics to 5-HT_1c and 5-HT₂ sites: clozapine potently interacts with 5-HT_1C sites. Eur J Pharmacol 191: 93-96.[CrossRef][Medline]

Carlsson A, Waters N, and Carlsson ML (1999a) Neurotransmitter interactions in schizophrenia-therapeutic implications. Biol Psychiatry 15: 1388-1395.

Carlsson ML, Martin P, Nilsson M, Sorensen SM, Carlsson A, Waters S, and Waters N (1999b) The 5-HT2A receptor antagonist M100907 is more effective in counteracting NMDA antagonist-than dopamine agonist-induced hyperactivity in mice. J Neural Transm 106: 123-129.[CrossRef][Medline]

Cremers TIFH, Giorgetti M, Bosker FJ, Hogg S, Arnt J, Mork A, Honig G, Bogeso KP, Westerink BHC, den Boer H, et al. (2004) Inactivation of 5-HT_2C receptors potentiates consequences of serotonin reuptake blockade. Neuropsychopharmacology 29: 1782-1789.[CrossRef][Medline]

Di Matteo V, De Blasi A, Di Giulio C, and Esposito E (2001) Role of 5-HT2C receptors in the control of central dopamine function. Trends Pharmacol Sci 22: 229-232.[CrossRef][Medline]

Eberle-Wang K, Lucki I, and Chesselet MF (1996) A role for the subthalamic nucleus in 5-HT_2C-induced oral dyskinesia. Neuroscience 72: 117-128.[CrossRef][Medline]

Eberle-Wang K, Mikeladze Z, Uryu K, and Chesselet MF (1997) Pattern of expression of the serotonin_2c receptor messenger RNA in the basal ganglia of rats. J Comp Neurol 384: 233-247.[CrossRef][Medline]

Frank MG, Stryker MP, and Tecott LH (2002) Sleep and sleep homeostasis in mice lacking the 5-HT_2c receptor. Neuropsychopharmacology 27: 869-873.[CrossRef][Medline]

Freed WJ, Bing LA, and Wyatt RJ (1984) Effects of neuroleptics on phencyclidine (PCP)-induced locomotor stimulation in mice. Neuropsychopharmacology 23: 175-181.

Geyer MA, Krebs-Thomson K, Braff DL, and Swerdlow NR (2001) Pharmacological studies of prepulse inhibition models of sensorimotor gating deficits in schizophrenia: a decade in review. Psychopharmacology 156: 117-154.[CrossRef][Medline]

Geyer MA, Swerdlow NR, Mansbach RS, and Braff DL (1990) Startle response models of sensorimotor gating and habituation deficits in schizophrenia. Brain Res Bull 25: 485-498.[CrossRef][Medline]

Herrick-Davis K, Grinde E, and Teitler M (2000) Inverse agonist activity of atypical antipsychotics drugs at human 5-hydroxytryptamine 2C receptors. J Pharmacol Exp Ther 295: 226-232.[Abstract/Free Full Text]

Kehne JH, Baron BM, Carr AA, Chaney SF, Eklands J, Feldman DJ, Frank RA, van Giersbergen PLM, McCloskey TC, Johnson MP, et al. (1996) Preclinical characterization of the potential of the putative atypical antipsychotic MDL 100907 as a potent 5-HT_2A antagonist with a favorable CNS safety profile. J Pharmacol Exp Ther 277: 968-981.[Abstract/Free Full Text]

Leysen JE, Gommeren W, Van Gompel P, Wynants J, Janssen PF, and Laduron PM (1985) Receptor-binding properties in vitro and in vivo of ritanserin: a very potent and long acting serotonin-S2 antagonist. Mol Pharmacol 27: 600-611.[Abstract]

Leysen JE, Niemegeers CJE, Tollenaere JP, and Laduron PM (1978) Serotonergic component of neuroleptic receptors. Nature (Lond) 272: 168-171.[CrossRef][Medline]

Mansbach RS, Geyer MA, and Braff DL (1988) Dopaminergic stimulation disrupts sensorimotor gating in the rat. Psychopharmacology 94: 507-514.[CrossRef][Medline]

Marion S, Weiner DM, and Caron MG (2004) RNA editing induces variation in desensitization and trafficking of 5-hydroxytryptamine 2C receptor isoforms. J Biol Chem 279: 2945-2954.[Abstract/Free Full Text]

Meltzer HY, Arvanitis L, Bauer D, Rein W, and Meta-Trial Study Group (2004) Placebo-controlled evaluation of four novel compounds for the treatment of schizophrenia and schizoaffective disorder. Am J Psychiatry 161: 975-984.[Abstract/Free Full Text]

Meltzer HY, Kennedy J, Dai J, Parsa M, and Riley D (1995) Plasma clozapine levels and the treatment of L-DOPA-induced psychosis in Parkinson's disease. A high potency effect of clozapine. Neuropsychopharmacology 12: 39-45.[CrossRef][Medline]

Meltzer HY, Matsubara S, and Lee JC (1989) Classification of typical and atypical antipsychotic drugs on the basis of dopamine D-1, D-2, and serotonin₂ pK_i values. J Pharmacol Exp Ther 251: 238-246.[Abstract/Free Full Text]

Nordstrom AL, Farde L, and Halldin C (1993) High 5-HT2 receptor occupancy in clozapine treated patients as demonstrated by PET. Psychopharmacology 110: 365-367.[CrossRef][Medline]

Potkin SG, Shipley J, Bera RB, Carreon D, Fallon J, Alva G, and Keator D (2001) Clinical and PET effects of M100907, a selective 5-HT-2A receptor antagonist. Schizophrenia Res 49 (Suppl 1): 242.

Rinaldi-Carmona M, Congy C, Santucci V, Simiand J, Gautret B, Neliat G, Labeeuw B, Le Fur G, Soubrie P, and Breliere JC (1992) Biochemical and pharmacological properties of SR 46349B, a new potent and selective 5-hydroxytriptamine 2 receptor antagonist. J Pharmacol Exp Ther 262: 759-768.[Abstract/Free Full Text]

Roth BL, Ciaranello RD, and Meltzer HY (1992) Binding of typical and atypical antipsychotic agents to transiently expressed 5-HT_1C receptors. J Pharmacol Exp Ther 260: 1361-1365.[Abstract/Free Full Text]

Snyder S (1976) The dopamine hypothesis of schizophrenia: focus on the dopamine receptor. Am J Psychiatry 133: 197-202.[Abstract/Free Full Text]

The French Clozapine Study Group (1999) Clozapine in drug-induced psychosis in Parkinson's disease. Lancet 353: 2041-2042.[CrossRef][Medline]

The Parkinson Study Group (1999) Low-dose clozapine for the treatment of drug-induced psychosis in Parkinson's disease. N Engl J Med 340: 757-763.[Abstract/Free Full Text]

Vanover KE, Harvey SC, Son T, Risso Bradley S, Kold H, Makhay M, Veinbergs I, Spalding TA, Weiner DM, Andersson CM, et al. (2004) Pharmacological characterization of AC-90179: a selective 5-HT_2A receptor inverse agonist. J Pharmacol Exp Ther 310: 943-951.[Abstract/Free Full Text]

Weiner DM, Burstein ES, Nash N, Croston GE, Currier EA, Vanover KE, Harvey S, Donahue E, Hansen HC, Andersson CM, et al. (2001) 5-Hydroxytriptamine_2A receptor inverse agonists as antipsychotics. J Pharmacol Exp Ther 299: 268-276.[Abstract/Free Full Text]

Weiner DM, Vanover KE, Brann MR, Meltzer HY, and Davis RE (2003) Psychosis of Parkinson's disease: serotonin 2A receptor inverse agonists as potential therapeutics. Curr Opin Investig Drugs 4: 815-819.[Medline]

Wettstein JG, Host M, and Hitchcock JM (1999) Selectivity of action of typical and atypical anti-psychotic drugs as antagonists of the behavioral effects of 1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane (DOI). Prog Neuropsychopharmacol Biol Psychiatry 23: 533-544.[CrossRef][Medline]

This article has been cited by other articles:

				L. R. Gardell, K. E. Vanover, L. Pounds, R. W. Johnson, R. Barido, G. T. Anderson, I. Veinbergs, A. Dyssegaard, P. Brunmark, A. Tabatabaei, et al. ACP-103, a 5-Hydroxytryptamine 2A Receptor Inverse Agonist, Improves the Antipsychotic Efficacy and Side-Effect Profile of Haloperidol and Risperidone in Experimental Models J. Pharmacol. Exp. Ther., August 1, 2007; 322(2): 862 - 870. [Abstract] [Full Text] [PDF]

				K. E. Vanover, D. Robbins-Weilert, D. G. Wilbraham, T. G. K. Mant, D. P. van Kammen, R. E. Davis, and D. M. Weiner The Effects of Food on the Pharmacokinetics of a Formulated ACP-103 Tablet in Healthy Volunteers J. Clin. Pharmacol., July 1, 2007; 47(7): 915 - 919. [Full Text] [PDF]

				K. E. Vanover, D. Robbins-Weilert, D. G. Wilbraham, T. G. K. Mant, D. P. van Kammen, R. E. Davis, and D. M. Weiner Pharmacokinetics, Tolerability, and Safety of ACP-103 Following Single or Multiple Oral Dose Administration in Healthy Volunteers J. Clin. Pharmacol., June 1, 2007; 47(6): 704 - 714. [Abstract] [Full Text] [PDF]

				P. J. Harvey, X. Li, Y. Li, and D. J. Bennett Endogenous Monoamine Receptor Activation Is Essential for Enabling Persistent Sodium Currents and Repetitive Firing in Rat Spinal Motoneurons J Neurophysiol, September 1, 2006; 96(3): 1171 - 1186. [Abstract] [Full Text] [PDF]

This Article Abstract Full Text (PDF) All Versions of this Article: jpet.105.097006v1 317/2/910    most recent Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Similar articles in this journal